I know it might sound creepy, but have you been to the Catacombs of San Callisto or Catacombs of San Sebastiano? San Callisto is the largest of Rome’s bone-filled catacombs, and were founded in the second century AD. Lots of early popes are buried here.

San Sebastiano catacombs are said to have housed the bodies of the apostles Peter and Paul for a time. There are also tours here on which you can check out paintings of doves and fish, along with some pagan tombs.

There’s also the ancient Mamertine Prison, which sits beneath the little church of San Pietro in Carcere, near the Capitoline Hill. Spies and various enemies of the state were locked up down here – again, it’s quite creepy, but worth visiting for its interesting excavations and history.

Hope that helps – no doubt some other community members will come up with more suggestions!